Microfluidic pumping and dispensing of liquid chemical reagents is the subject of three U.S. Pat. Nos. 5,585,069; 5,593,838; and 5,603,351, all assigned to the David Sarnoff Research Center, Inc., and hereby incorporated by reference. The system uses an array of micron sized reservoirs, with connecting microchannels and reaction cells etched into a substrate. Electrokinetic pumps comprising electrically activated electrodes within the microchannels provide the propulsive forces to move the liquid reagents within the system. The electrokinetic pump, which is also known as an electroosmotic pump, has been disclosed by Dasgupta et al., see "Electroosmosis: A Reliable Fluid Propulsion System for Flow Injection Analyses", Anal. Chem. 66, pp 1792-1798 (1994). The chemical reagent solutions are pumped from a reservoir, mixed in controlled amounts, and then pumped into a bottom array of reaction cells. The array can be decoupled from the assembly and removed for incubation or analysis.
The above described microfluidic pumping can be used as a printing apparatus. The chemical reagent solutions are replaced by dispersions of cyan, magenta, and yellow pigment. The array of reaction cells may be considered a viewable display of picture elements, or pixels, comprising mixtures of pigments having the hue of the pixel in the original scene. When contacted with paper, the capillary force of wetting the paper fibers pulls the dye from the cells and holds it in the paper, thus producing a paper print, or photograph, of the original scene.
For printing a photographic quality image, it is desirable to print a continuous tone scale of colored inks. Such a continuous tone printing apparatus, based on the microfluidic printing as described, has been disclosed in the above cross referenced and commonly assigned copending U.S. patent application Ser. No. 08/868,426, filed concurrently herewith entitled "Continuous Tone Microfluidic Printing", by DeBoer, Fassler, and Wen. The disclosure of this related application is incorporated herein by reference. In U.S. patent application Ser. No. 08/868,426, a colorless ink is mixed with the colored ink mixtures to make colored inks of different degree of color saturation at each pixel, which is needed for a continuous tone image.
A problem with microfluidic printing is in the control of the amount of inks transferred from the printing apparatus to the receiver medium. During printing, the ink meniscus in the ink mixing pixel chambers are brought into contact with the receiver medium. The inks are absorbed by the receiver medium by action of the wetting of the fibers or pores in the receiver medium. Since the capillary force in the receiver medium is typically much stronger than the holding strength of the microchannels in the microfluidic printing apparatus, the ink transfer needs to be stopped at just the right time to prevent excess ink from being continually drawn from the microchannels in the microfluidic printing apparatus. The control of the ink transfer time is particularly difficult in conditions where the temperature may vary, because the rate of flow of the ink will be temperature sensitive. As it is well known to the persons skilled in the art, excessive ink transfer to the receiver medium typically causes severe coalescence or smearing of the ink on the receiver medium, which produces visible image artifacts and lowers the printing resolution Excess ink transfer also causes excess bleeding between inks of different colors which produces image defects and variabilities in color balance.